1. Field of the Invention
The present invention relates to an EGR controller of a cylinder-cut-off-engine which controls actions of exhaust gas recirculation (EGR) in a cylinder cut-off engine which can run with change over of the running condition between an all-cylinder operation with change over between all cylinders operating and a cut-off-cylinder run where some of the cylinders are cut off.
2. Description of the Related Art
A conventionally known type of the EGR controller for cylinder cut-off engine is such as on Japanese Patent Laying-Open Gazette Showa-60-45767 (1985). This cylinder cut-off engine is a four-cylinder engine which runs with change over between a cut-off-cylinder run where fuel supply to two of four cylinders is stopped as conditioned in regard to engine cooling water temperatures, revolving speeds and acceleration and deceleration states and where induction and exhaust valves of these two cylinders are maintained in a blocked state and an all-cylinder run where all the four cylinders run normally. Especially when the cooling water temperature is not higher than a threshold value To (60.degree. C.), the all-cylinder run is executed unconditionally or, when the cooling water temperature is not higher than the threshold value, the cut-off-cylinder run is not executed; that is, this threshold value To is a condition for change over between the cut-off-cylinder run and the all-cylinder run. The EGR controller controls the EGR actions that NOx in the exhaust gas is reduced by decrease in the combustion temperatures at the cylinder cut-off engine through recalculation of the exhaust gas to the induction side, and the EGR actions are controlled to different EGR rates as conditioned in regard to the engine running states, whether the cut-off-cylinder run or the all-cylinder run.
In general, if compared with a case of the cut-off-cylinder run at the same cooling water temperatures, when the all-cylinder run is executed at low temperature range where the cooling water temperature is lower than the threshold value To, such as 40.degree. C., increase in the pumping loss and decrease in the charging efficiency cause decrease in the combustion temperatures to result in decrease in the NOx generation during the run but increase in the fuel adhesion on the combustion chamber, and thus the cylinder cut-off engine tends to generate unburnt gas. Moreover, since the conventional EGR controller executes the EGR actions on the cylinder cut-off engine at the all-cylinder run even in such low temperature range as described above, the combustion temperature decreases the further and thus the above-described problem becomes more obvious.
Further, if the running state of the cylinder cut-off engine is changed over between the cut-off-cylinder run and the all-cylinder run, the engine combustion state and the exhaust gas are likely to be unstabilized. Since the above-described conventional EGR controller for cylinder cut-off engine only modifies the EGR rate at the same time as the running state changeover of the cylinder cut-off engine, unstableness of the combustion state at the above-described transition period increases. That is, in the state immediately after changeover from the all-cylinder run to the cut-off-cylinder run, since the combustion temperature increases with the enhanced filling efficiency at the cylinders that continue running, the w combustion state becomes temporarily unstable. The EGR actions, if executed for these cylinders in the unstable combustion state, will increase the unstableness of the combustion state. On the contrary, when the EGR actions are executed at the cylinder cut-off engine while the cut-off-cylinders are running, the combustion temperature has been decreased inside the running cylinders and, if the cut-off-cylinder run is changed over into the all-cylinder run in this state, the combustion state also becomes temporarily unstable since the combustion temperature decreases further along with decrease in the filling efficiency. If the EGR actions are executed in this unstable combustion state, the unstableness of the combustion state inside the cylinders will also be increased.